The field of the disclosure relates generally to a drive circuit for electric motors and, more specifically, a drive circuit that operates two or more electric motors in parallel with a single inverter or single drive device. The electric motors may be permanent magnet (PM) motors or induction motors.
PM electric motors are operated synchronously in that the rotor turns at a speed that matches the speed at which a rotating magnetic field generated by the stator turns. The stator and rotor of a PM motor, or synchronous motor, are energized independently, generally with an alternating current (AC) supplied to the stator windings. In contrast, induction motors operate asynchronously, i.e., the rotor turns at a speed that lags the speed of the stator rotating magnetic field, i.e., the synchronous speed. The relative speed of the rotor and the rotating magnetic field induces the rotor current.
Generally, PM motors are more efficient, but tend to be more complex and often costlier than their counterpart induction motors. At least some PM motors are driven utilizing a vector control scheme that independently monitors and controls motor torque and motor flux, i.e., monitors rotor position and phase currents, and independently controls torque current and flux current via a complex voltage (i.e., a voltage amplitude and phase represented in a complex plane). For a given PM motor, torque current and flux current are controlled over time by a pulse width modulated (PWM) signal that controls switching in an inverter that supplies, for example, three-phase current to the stator windings. Such control may be accomplished, for example, using vector control. A three-phase PWM voltage signal for energizing the stator windings is generated based on a complex voltage vector in a rotating rotor reference frame. The complex voltage vector is derived using, for example, a vector control algorithm executing on a digital signal processor (DSP) or other suitable processor for controlling the inverter.
Vector control algorithms are generally known. An exemplary vector control algorithm begins with measured stator phase currents that are transformed to the rotating rotor reference frame. The rotating rotor reference frame is derived from the rotor position, which is either measured directly or integrated from a measured rotor speed or inferred through mathematical models. For each phase, a rotor flux linkage vector is estimated based on the stator current vector and the magnetizing inductance of the stator coil. The rotor flux linkage vector gives a rotor angle that enables the stator current vector to be converted to a (d,q) coordinate system in the rotating rotor reference frame. The (d,q) coordinate system, sometimes referred to as the flux-torque coordinate system, represents a complex current vector with orthogonal components along a direct axis (d) and a quadrature axis (q) such that a field flux linkage component of the complex current vector aligns with the d-axis and a torque, or armature flux, component aligns with the q-axis. Once the stator current vector is represented in the (d,q) coordinate system, its components may be controlled using traditional scalar control, including, for example, proportional and integral (PI) control, that produce a complex commanded voltage vector in the (d,q) coordinate system. The complex commanded voltage vector is then converted back to the original rotating rotor reference frame and is the basis for generating a PWM voltage signal for controlling an inverter that energizes the stator windings.
At least some motor applications can utilize multiple, smaller and more efficient motors in parallel to improve output or efficiency. Such applications may include heating, ventilation, and air conditioning (HVAC), refrigeration, compression, pumps, or other fluid-moving equipment, as well as electric drives for wheels, gears, belts, or other mechanical loads. Induction motors are often utilized in such applications due to their relative simplicity and ability to operate asynchronously, i.e., to allow “slip” between rotor rotation and magnetic field rotation, thereby simplifying loading of each motor. Conversely, each PM motor in a multi-motor application typically requires a dedicated PM drive to generate the appropriate PWM signal to operate the motor synchronously for its particular load. Consequently, each PM drive is generally rated for full output power required for the application, resulting in higher costs, lower efficiency, and more complex configuration and installation. Alternatively, and notably less practical, multiple PM motors or induction motors may be combined in parallel with a single drive, but is generally impractical, because loading on the various motors is not known or controlled well enough to balance loads among the parallel motors. Consequently, in such applications, the motors operate at varying speeds under varying loads that could lead to stability challenges and motor damage, if they operate at all. Furthermore, connecting and operating multiple PM motors on a single inverter that must function as the same synchronous speeds present much more difficulty as variability in loading will cause instabilities and finally loss of synchronism in the system.